An antenna transmits/receives electromagnetic wave by outputting certain radiation pattern, and realizes generally only one frequency band. However, it has been required to realize a plurality of frequency bands in recent, and so multi band dual polarization antenna shown in following FIG. 1 has been developed.
FIG. 1 is a perspective view illustrating schematically common multi band dual polarization antenna, and FIG. 2 is a view illustrating beam pointing error of the antenna in FIG. 1.
In FIG. 1, a multi band dual polarization antenna includes a reflection plate 100, first radiators 102 and second radiators 104.
The first radiators 102 are formed on the reflection plate 100, are used for low frequency band, and generate dual polarized wave (±45° polarized waves).
The second radiator 104 locates inside the first radiator 102, is used for high frequency band, and generates dual polarized wave (±45° polarized waves).
Center of a main beam of a radiation pattern in the antenna shifts along θ axis in accordance with change of tilting angle of the antenna as shown in FIG. 2(A) in case that the main beam is normally outputted. However, the center of the main beam shifts really along a beam pointing line 200 as shown in FIG. 2(B). As a result, beam pointing error occurs by an angle 202.
Beam tracking error means difference of horizontal gain of ±45° polarized waves. The beam tracking error increases also according as the tilting angle of the antenna is changed.
That is, the antenna may not output the radiation pattern in the desired direction due to the beam pointing error and the beam tracking error.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.